is a simple and fast candidate construction of a PRF from an MD-style hash function which applies the keyed hash function and then a cheap, un-keyed output transform such as truncation. Spurred by its use in the widely-deployed Ed25519 signature scheme, this paper investigates the provable PRF security of to deliver the following three-fold message: (1) First, we prove PRF security of . (2) Second, we show that has a quite unique and attractive feature, namely that its multi-user security is essentially as good as its single-user security and in particular superior in some settings to that of competitors. (3) Third, it is technically interesting, its security and analysis intrinsically linked to security of the compression function in the presence of leakage.